It is known that such a device, for example of TAWS type (“Terrain Avoidance and Warning System”) or of GPWS type (“Ground Proximity Warning System”) is aimed at detecting any risk of collision of the aircraft with the surrounding terrain and at warning the crew when such a risk is detected, so that the latter can then implement a terrain avoidance maneuver. Such a device generally comprises:                a first means knowing the profile of the terrain at least in front of the aircraft;        a second means for determining an avoidance trajectory of the aircraft;        a third means connected to said first and second means, for verifying whether there exists a risk of collision of the terrain for the aircraft; and        a fourth means for issuing an alarm signal, in case of detection of a risk of collision by said third means.        
Generally, said second means determines the avoidance trajectory (which is taken into account by the third means so as to detect a risk of collision with the terrain), by using a slope exhibiting a fixed and invariable value, in general 6° for a transport plane, regardless of the type of aircraft and regardless of its actual performance.
Of course, such a mode of calculation exhibits the risk of underestimating or overestimating the actual performance of the aircraft, this possibly causing overly late detections of risks of collision or false alarms. This mode of calculation is therefore not completely reliable.
Document EP-0 750 238 discloses a terrain avoidance device of the aforesaid type. This known device makes provision to determine two trajectories which are subsequently compared with the profile of the terrain overflown, one of said trajectories representing the predicted effective trajectory of the aircraft and the other trajectory possibly corresponding in particular to a predicted climb trajectory. This prior document makes provision to take account of maneuvering capabilities of the aircraft to predict these trajectories, without however indicating the way in which these trajectories are actually calculated or predicted.